1. Field of the Invention
The present invention generally relates to a crystallization apparatus, crystallization method, and phase shifter applied to a non-single-crystal semiconductor film such as a polycrystalline or amorphous semiconductor film, particularly to a crystallization apparatus, crystallization method, and phase shifter for modulating a phase of laser light to be applied to the non-single-crystal semiconductor film in the crystallization.
2. Description of the Related Art
Materials of thin film transistors (TFT) for use as switching devices for controlling voltages to be applied to pixels, for example, of a liquid crystal display (LCD) have heretofore been roughly divided into amorphous silicon and poly-silicon.
The mobility of the poly-silicon is higher than that of the amorphous silicon. Therefore, when the poly-silicon is used to form thin film transistors, a switching speed increases and the display responds more quickly as compared with the use of the amorphous silicon. Such thin film transistors are also usable as components of peripheral LSI circuits. Furthermore, there is an advantage that design margins of other components can be reduced. When peripheral circuits such as a driver circuit and DAC are integrated on the display, these peripheral circuits are operable at a higher speed.
Although the poly-silicon includes a number of crystal grains, the mobility thereof is lower than that of single-crystal silicon. When the poly-silicon is used to form a small-sized transistor, this raises a problem that the number of crystal grain boundaries fluctuates within a channel region. In recent years, crystallization methods for producing single-crystal silicon grains of a large diameter have been proposed in order to improve the mobility and reduce the fluctuation in the number of crystal grain boundaries within the channel region.
For this type of crystallization method, “phase-modulated Excimer Laser Annealing (ELA)” has heretofore been known that applies excimer laser light to a non-single-crystal semiconductor film via a phase shifter (phase-shift mask) disposed in parallel with and in the proximity of the semiconductor film to produce a crystallized semiconductor film. Details of the phase-modulated ELA are disclosed, for example, in “Applied Surface Science Vol. 21, No. 5, pp. 278 to 287, 2000”.
In the phase-modulated ELA, the light intensity distribution of the light applied to the non-single-crystal semiconductor film is controlled in a zone corresponding to a phase-shift section of the phase shifter to have an inverse peak pattern (that is, a pattern in which the light intensity significantly increases according to an increase in the distance from the center of the zone). As a result, a temperature gradient is generated in the semiconductor film of a molten state according to the light intensity distribution, and a crystal nucleus is created at a part of the semiconductor film which first coagulates according to the light intensity of substantially 0. Then, a crystal grows in a lateral direction toward the outside from the crystal nucleus (lateral growth), thereby forming a single-crystal grain of a large diameter.
Conventionally, the phase shifter for general use is a so-called linear phase shifter, which includes pairs of rectangular regions having a phase retardation π (180 degrees) therebetween and repeatedly arranged in one direction. In this case, a straight boundary between two regions serves as the phase-shift section, and therefore the light intensity on the non-single-crystal semiconductor film is controlled to have an inverse peak pattern in which the light intensity is substantially 0 at a location of an axis corresponding to the phase-shift section and one-dimensionally increases according to an increase in the distance from the location.
In the conventional art in which the aforementioned linear phase shifter is used, a temperature distribution is lowest on an axis corresponding to the phase-shift section, and a temperature gradient is generated in a direction perpendicular to the axis corresponding to the phase-shift section. That is, the crystal nucleus is created on the axis corresponding to the phase-shift section, and crystallization proceeds from the crystal nucleus in the direction perpendicular to the axis corresponding to the phase-shift section. As a result, the crystal nucleus is created on the axis corresponding to the phase-shift section, but a position on the axis where the crystal nucleus is created is indefinite. In other words, in the conventional art, it has been impossible to specify the creation point of the crystal nucleus, and it has also been impossible to two-dimensionally control a region where the crystal grain is formed.